Digital cameras sample the original light that bounces off a subject to create a digital image. A digital camera may use a shutter in combination with a sensor to determine an exposure time and acquire the proper amount of light to make a good image. The proper amount of light may come from accumulating ambient light over time until the proper amount of light is obtained. Alternatively, the addition of artificial light from a flash speeds up the amount of time, until the proper amount of light is obtained to make a good image.
In a typical mechanical shutter, the timing between scan lines of a sensor and the actuation of the shutter is such that the shutter is closed until the sensor (such as a complementary metal-oxide semiconductor (CMOS) sensor, or a charge coupled device (CCD) sensor) is ready to be exposed to the light in the picture frame. When the mechanical shutter opens, nearly 100% of the light is allowed to pass and be sensed by the light sensitive pixels in the sensor. When the mechanical shutter gives the command to close and the shutter closes, the light no longer reaches the sensor.
Some digital cameras have a mechanical shutter to control the exposure time of the CMOS sensor. Unfortunately, the inclusion of a mechanical shutter adds significant expense to the cost of a camera. Further, a mechanical shutter due to size and dimensions adds physical bulkiness to the camera.
Some cameras may have sensors implementing a global shutter timing mechanism for the light sensitive pixels. These sensors have electronic components added to each pixel location in the sensor. Typically, in a sensor implementing a global shutter timing mechanism, all the pixels begin and end their integration phase at the same time, and the intensities of all the pixels are simultaneously transferred to a light insensitive storage area at the same time. Sensors employing a global shutter timing mechanism, therefore, typically have little to no potential for motion artifacts.
A typical single chip CMOS image sensor 199 is illustrated by the block diagram of FIG. 1. Pixel array 190 includes a plurality of pixels arranged in a predetermined number of columns and rows.
Generally, the rows of pixels in array 190 are read out one by one. Accordingly, pixels in a row of array 190 are all selected for readout at the same time by a row select line, and each pixel in a selected row provides a signal representative of received light to a readout line in its column. In array 190, each column also has a select line, and the pixels of each column are selectively read out, in response to the column select lines.
The row lines in pixel array 190 are selectively activated by a row driver 191, in response to row address decoder 192. The column select lines are selectively activated by a column driver 193, in response to column address decoder 197. The pixel array is operated by the timing and control circuit 195, which controls address decoders 192, 197 for selecting the appropriate row and column lines for pixel signal readout.
The signals on the column readout lines, typically, include a pixel reset signal (Vrst) and a pixel image signal (Vsig) for each pixel. Both signals are read into a sample and hold circuit (S/H) 196, in response to column driver 193. A differential signal (Vrst−Vsig) is produced by differential amplifier (AMP) 194 for each pixel, and each pixel's differential signal is amplified and digitized by analog-to-digital-converter (ADC) 198. The ADC 198 supplies the digitized pixel signals to an image processor 189, which can perform appropriate image processing before providing digital signals defining the image.
An electronic shutter for image sensors has been developed to serve in place of a mechanical shutter. The electronic shutter controls the amount of photo-generated charge accumulated by a pixel cell by controlling the integration time of the pixel cell. This feature is especially useful when imaging moving objects, or when the image sensor itself is moving and a shortened integration time is necessary for quality images.
Typically, a pixel cell having an electronic shutter includes a shutter transistor and a storage device, which is typically a pn-junction capacitor. The storage device stores a voltage representative of the charge generated by a photo-conversion device in the pixel cell. The shutter transistor controls when, and for how long, charge is transferred to the storage device. This controls the integration time of the pixel cell.
There are two typical modes of operation for an electronic shutter, namely, rolling shutter and global shutter. When an electronic shutter operates as a rolling shutter, each row of pixels in the array integrates photo-generated charge, one at a time, and each row is read out one at a time. When an electronic shutter operates as a global shutter, all pixels in the array integrate photo-generated charge simultaneously, and each row is read out one at a time.
Global shuttering provides advantages over row shuttering. Essentially, global shuttering is able to provide a “snap shot” of the imaged object. Consequently, global shuttering offers increased accuracy of an imaged object, with uniform exposure time and uniform image content.
On the other hand, because the pixel cells of the pixel array are read out row by row, pixel cells in a row, that are read out last, must store photo-generated charge in their respective storage devices longer, than pixel cells in rows that are read out earlier. These storage devices may lose charge over time, and the longer these storage devices must store photo-generated charge, there is more charge that is lost. Therefore, charge loss is especially problematic for pixel cells in the last read row. When charge is lost by a pixel cell, the resultant image may have a poor quality, or may be distorted.
Additionally, a common problem associated with active pixel imager cells, when operated in global shutter modes, is that storage devices, or storage nodes (SNs), accumulate parasitic charges while holding charge transferred from each photodiode (PD). This lowers the global shutter pixel efficiency and results in appearance of vertical shading; a moving object may also have smears and shadings. Accordingly, what is needed is a pixel cell with an electrical shutter having improved charge transfer efficiency, minimal charge loss, and reduced accumulation of parasitic charges.